Oxygen enriched air, eg from a membrane-type oxygen/nitrogen separator 16, is supplied to the intake of a compression-ignition engine to ensure spontaneous combustion of gaseous fuels without further engine modification. A catalyst 44, eg a platinum group metal catalyst on a ceramic substrate 40, may be coated on the piston crown 34 and cylinder head 48 to accelerate the rate, and reduce the temperature, of the reaction between the fuel and the oxygen. The concentration of oxygen in the intake air may be controlled by a valve 20 which selectively admits atmospheric air to the oxygen-enriched air from the separator 16. Valve 20, together with fuel gas supply valve 6, may be controlled according to sensed operating conditions. The invention is applicable to automotive engines or stationary engines for electrical power generation.; In the latter case, the exhaust gas may be used to produce steam to drive a further electrical generating system.